In fantasy, guns are either so realistically overpowered, they make swords and bows obselete, or they're so archaic, imprecise, and unreliable that you need an entire class focused on them just to make them any useful. Either way, it ends up looking like you threw Gandalf an AK-47, ruining the theme and tone of your game.
I've created some rules for fantasy firearms, which function enough like guns... [click here for more]